Recently, there has been an increasing demand to save energy of an entire driving system by using an inverter to operate a motor at variable speeds. Although the efficiency of an entire system can be increased in various operation states by driving an induction motor with an inverter, a carrier harmonic component generated by the inverter causes a loss in rated operation, unlike a case in which the system is driven by using commercial power supply. This type of loss may lower the efficiency.
A technology to improve the efficiency of an inverter-driven induction motor is described in Patent Literature 1, in which a bar of a rotor is shaped so that the bar is enlarged toward the outer circumference of the rotor. This enables an increase in secondary resistance due to the enlarged skin effect and thereby reduces the harmonic copper loss of rotor cage. Another technology is described in Patent Literature 2, in which a rotor slot is formed wider than the conventional shape and is disposed near the outer circumference of the rotor while being inclined in a rotational direction with respect to a straight line drawn from the center of the rotor to the outer circumference. In a technology in Patent Literature 3, to improve start characteristics and the efficiency in a rated operation, a rotor conductor has a double-cage shape; where an upper bar is displaced by a predetermined angle in the rotational direction or the direction opposite to the rotational direction.